Charting 500 years of Mississippi floods

by Mary Caperton Morton
Tuesday, July 3, 2018

The Mississippi River is notorious for flooding its muddy banks, with many critical areas actively managed by the Army Corps of Engineers to control flooding. But despite all the controls, major floods still occur. A new study looking at flood trends over the last 500 years suggests that manmade flood controls may be responsible for most of the increase in flood magnitudes on the river.

“The floods that we’ve had over the last century are bigger than anything [else] we’ve seen in the last 500 years,” said lead author Sam Muñoz, of Northeastern University in Boston, in a statement. And over the past 150 years, the magnitude of a 100-year flood — a flood with a 1 percent chance of occurring in any given year — has increased by 20 percent, according to the new study in Nature.

To establish a timeline for the river’s flood history before and after human intervention, Muñoz and colleagues from Woods Hole Oceanographic Institution in Massachusetts, extracted cores of bottom sand, silt and sediment from three oxbow lakes adjacent to the Mississippi River: Lake Mary in Mississippi and False River Lake and Lake St. John in Louisiana. By analyzing the sizes of sediment grains carried into the lakes by floods, the team charted flood histories going back 500 years — 350 years longer than previous studies. Isotopic and luminescence dating techniques illuminated a timeline for the flood events, which the researchers then compared to records of naturally oscillating climate cycles that affect sea-surface temperatures and control rainfall over the Mississippi Basin. They found that the Mississippi’s flood cycles corresponded with ocean and climate cycles.

“We’re able for the first time to really parse out how the natural variability of the climate system influences flooding, and then how people have modified that,” Muñoz said. The team concluded that although the river does rise and fall with North Atlantic climate fluctuations, natural variation only accounts for a quarter of the increase in flood magnitude. The remaining 75 percent is due to human modifications to the Mississippi River and its basin.

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